Recently, a trend toward miniaturization and slimness in portable electronic instruments such as mobile phones, notebook computers, PDAs and the like has raised a need for realization of high capacities in lithium secondary batteries which are used as an energy source for such portable instruments. Lithium/cobalt complex oxides, commercially available at present, can be relatively easily synthesized and exhibit superior safety and charge/discharge cyclability, but have reached their uppermost limit of capacity, thus presenting problems associated with limitations to achieve higher capacity of batteries.
As such, a lithium/manganese complex oxide (LiMnO2 or LiMn2O4) using inexpensive manganese (Mn) or a lithium/nickel complex oxide using nickel (Ni) thus receives a great deal of attention as a replacement for a lithium/cobalt complex oxide. Among these, the lithium/manganese complex oxide having a layered-structure is advantageous in that it has a much higher capacity than the lithium/cobalt complex oxide, but suffers from an unstable structure and thus poor charge/discharge cyclability. Whereas, a spinel lithium/manganese complex oxide has an excellent thermal stability, but exhibits a lower capacity than the lithium/cobalt complex oxide, thus leading to difficulty in application thereof to high-capacity batteries.
Meanwhile, the lithium/nickel complex oxide is a high-capacity material, but suffers from problems such as inferior charge/discharge cyclability and difficulty in preparation thereof. In order to solve such disadvantages, Japanese Patent Publication No. Hei 8-213015 has proposed LixNiaCObMcO2 wherein x is in a range of 0.8 to 1.2, a is in a range of 0.01 to 0.99, b is in a range of 0.01 to 0.99, c is in a range of 0.01 to 0.3, the sum of a, b and c is in a range of 0.8 to 1.2, and M is at least one element selected from the group consisting of Al, V, Mn, Fe, Cu and Zn, as a cathode active material having excellent self-discharge characteristics and excellent charge/discharge cyclability and capable of maintaining battery performance favorable for storage and use under high temperature conditions. The lithium/nickel complex oxide obtained using the method disclosed in the above Japanese Patent exhibits relatively high capacity compared to the lithium/cobalt complex oxide and has improved charge/discharge cyclability to a degree, but still has unsatisfactory high-temperature characteristics when in a charged state. As a result, such a lithium/nickel complex oxide initiates decomposition accompanied by release of gas such as oxygen, and the thus released gas such as oxygen reacts with an electrolyte, or nickel ions react with the electrolyte, thus leading to an elevated internal pressure of the battery which in turn results in the battery swelling or in the worst case a risk of the battery exploding.
On the other hand, in order to improve thermal stability of the battery, where it is desired to use the lithium/nickel complex oxide in which a portion of nickel (Ni) is replaced with other metal species as the cathode active material, partial replacement of nickel with the added metal can improve thermal stability, but the capacity of the battery is significantly decreased as the amount of the metal added is increased.
As such, there is a need in the art for the development of a cathode active material which has a high capacity for use in preparation of a high-capacity battery and which is also thermally stable at high temperatures, thus capable of inhibiting reaction with the electrolyte.